1. Field of the Invention
The present invention relates to surfaces used to reflect light, and particularly to highly light reflectant surfaces that provide even diffusion of light energy from their surfaces.
2. Description of Related Art
Special light reflectant surfaces are used in a variety of applications requiring light energy to be almost completely reflected while providing an even distribution of light from the surface. While good mirrored surfaces can provide nearly perfect reflectivity of visible light, the light energy exiting these surfaces does so only at an angle equal to the incident angle of light contact. For many applications it is important that light be reflected with an even distribution of light from the surface. This latter property is referred to as diffuse or "lambertian" reflectance.
For instance, projection screens, such as those used for slide or motion picture presentations, must provide both high reflectivity and a light diffusion/distribution over a sufficiently wide field so as to provide a clear image to most of an audience. Many reflectant screens employ a coating of glass beads or similar material as a reflectant aid to provide excellent reflectivity over a defined projection field (e.g., approximately 20.degree. from a center line), with significantly diminished reflectivity outside of the defined projection field. These screens provide very good viewing within the defined field, and are less prone to interference from stray light sources other than the primary light source aimed perpendicular to the screen. In order to provide better viewing to a wider defined field, matte-finished screens are effective at providing a more even light distribution to an entire audience. Although hardly critical for most projection screen applications in darkened rooms, in either instance it is important that the screen absorb or transmit as little light as possible so as to assure maximum reflective image to the audience.
Reflectivity is far more critical in many other applications. For instance, displays used in electronic equipment (e.g., instrument panels, portable computer screens, liquid crystal displays (LCDs), etc.), whether relying on supplemental lights (e.g., backlight) or merely ambient light, require very good diffuse reflectant back surfaces to maximize image quality. Reflectivity is particularly critical with backlighted displays in battery powered equipment, where better reflectivity is directly related to smaller required light sources and resulting lower power demands.
A substantial but demanding light reflective market is the one for portable or laptop computer displays. This market is demanding because it requires high levels of diffuse reflection from very thin materials. For this market it is critical that the backlight reflector is relatively thin, i.e., less than 0.25 mm and preferably less than 0.15 mm, to minimize the thickness of the completed display assembly.
Due to the many different applications that exist for reflectant materials, it is not surprising that there are many different commercially available products with a variety of diffuse reflective properties. Until the present invention, the best known diffuse reflective material with excellent diffuse reflectivity was that described in U.S. Pat. No. 4,912,720 and sold under the trademark SPECTRALON by Labsphere, Inc., North Sutton, N.H. This material comprises lightly packed granules of polytetrafluoroethylene that has a void volume of about 30 to 50% and is sintered into a relatively hard cohesive block so as to maintain such void volume. Using the techniques taught by U.S. Pat. No. 4,912,720, it is asserted that exceptionally high diffuse visible light reflectance characteristics can be achieved with this material, with reflectance over previously available reflectant material increasing from 97% to better than 99%. Despite the advantages of the SPECTRALON material it does not appear to be available in very thin films of less than 0.25 mm, such as those needed for the laptop display markets, and furthermore at these thickness levels it is believed that adequate reflection performance can not be obtained.
One of the materials commonly used as a diffuse reflector in backlight reflectors applications is sold under the trade name of MELINEX.RTM. polyester films by ICI Films of Wilmington, Del. These materials are available in the desirable thickness range of less than 0.15 mm. However the maximum visible light reflectance of 0.13 mm thick material is only 88.4%. Furthermore, in measuring the diffuseness of this material, the diffusivity is much less than desired. Thicker films of this material, such as 0.25 mm thick material, achieve only a maximum visible reflectance of 94.3% while suffering from added thickness and stiffness.